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Architectural, petrophysical and hydromechanical properties of fault zones in fractured-porous rocks: compared studies of a moderate and a mature fault zones (France).
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Architectural, petrophysical and hydromechanical properties of fault zones in fractured-porous rocks: compared studies of a moderate and a mature fault zones (France).
Relations entre structure, propriétés pétrophysiques et hydromécaniques des zones de failles dans les roches poro-fracturées : étude de deux zones de failles modérée et mature (France).
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... The study site was located in the GAS-gallery (Figure 1), ~250m below the surface topography, which enables to set up experiments in subsurface conditions, at least in terms of hydrostatic stress, Guglielmi et al., [2015] estimated the main stress to 6MPa. The gallery gives direct access to the 30° southward-dipping layers, and five vertical fully-cored boreholes (~20m deep) named P1 to P5 were drilled in 2009, and were studied as part of previous works (Jeanne 2012;Jeanne, Guglielmi, and Cappa 2013;Jeanne, Guglielmi, and Cappa 2012;). ...
... The studied formations are located within vadose zone of "Fontaine de Vaucluse" aquifer system. LSBB (Low Noise Underground Laboratory) GAS-gallery gives access to the 30° southward-dipping layers, and five vertical fully-cored boreholes (~20m deep) were drilled as part of previous works (Jeanne 2012;Jeanne, Guglielmi, and Cappa 2013;Jeanne, Guglielmi, and Cappa 2012;. ...
... This suggests that at meter-scale, fractures start to be sampled but do not dominate the signal, which enables to capture both signatures of the matrix and the fractures. In addition, the faults present in the LSBB area(Jeanne 2012), and which did not affect our measurements (no fault between the tested boreholes), but which were unavoidably measured by, may have played a role even bigger in lowering the velocities than fractures-sets alone. For exampleAl-Harrasi et al. (2011) related the anisotropy in the Natih carbonate reservoir (Oman) the rock type and proximity to major faults. ...
Physical properties of carbonate rocks cannot be fully captured from laboratory-sized samples. Indeed, heterogeneous facies distribution and/or diagenetic alterations may lead to significant variations in petrophysical properties within few meters. In carbonates, diagenetic
transformations are tightly related to nature of fluids flowing through the formations, e.g. via fractures network. Consequently, reservoir properties may have patchy distribution, and may not be correlatable (e.g. using facies distribution or wells-logs correlations) within few meters.
Our works aim at characterizing carbonates anisotropy at different scales, and are subject of two presentations at SEG’s 87 th Annual Meeting. This abstract deals with the first part of our approach, that’s to say characterizing impact of diagenetic alteration on reservoir properties and seismic anisotropy, from meter to multi-meter scale. Crosshole acoustic-survey has been carried-out using ultrasonic frequencies (50kHz), and vertical-resolution of 10cm. The main conclusions are: (1) multi-meter crosshole surveys, enable measuring elastic properties of both matrix, and fractured-matrix. Wave velocities measured at multi-meter scale are comparable to that from laboratory measurements, and this analogy is attributed to poor likelihood of finding dense fractures-sets –which would drastically change elastic moduli– at reduced scale. (2) Weak anisotropy is detected, varying between +10% (slow axis perpendicular to strata) and –2% (slow axis parallel to strata), and likely results from combination of matrix anisotropy (mineral orientation and pore geometry) and fractures effects. (3) In heterogeneous layered formations, variation of mean velocity with source and receivers locations significantly impact anisotropy parameters estimated by curve fitting. One should be aware of this potential bias, while interpreting crosshole surveys with sparse data, especially in carbonate formations.
... The studied formations are located within vadose zone of "Fontaine de Vaucluse" aquifer system. LSBB (Low Noise Underground Laboratory) GAS-gallery gives access to the 30° southward-dipping layers, and five vertical fully-cored boreholes (~20m deep) were drilled as part of previous works (Jeanne 2012;Jeanne, Guglielmi, and Cappa 2013;Jeanne, Guglielmi, and Cappa 2012;. The present study focuses on analyzing elastic properties of six 5" cores (C01 to C06), which are representative of different layers intersected by the boreholes (Figure 1). ...
Physical properties of carbonate rocks cannot be fully captured from laboratory-sized samples. Indeed, heterogeneous facies distribution and/or diagenetic alterations may lead to significant variations in petrophysical properties within few meters. In carbonates, diagenetic transformations are tightly related to nature of fluids flowing through the formations, e.g. via fractures network. Consequently, reservoir properties may have patchy distribution, and may not be correlatable (e.g. using facies distribution or wells-logs correlations) within few meters. Our works aim at characterizing carbonates anisotropy at different scales, and are subject of two presentations at SEG's 87th Annual Meeting. This abstract deals with the second part of our approach, that's to say characterizing impact of diagenetic alteration on reservoir properties and seismic anisotropy, from centimeter to multi-meter scale. This part of the works integrate data from centimeter-scale (mini-cores), decimeter-scale (5″ cores), multi-meter (ultrasonic crosshole), and hectometer-scale (seismic), which have been measured at suitable frequency ranges (1MHz, 250kHz, 50kHz, and 1–100Hz, respectively). Although anisotropy is measureable at every scales, its origins vary according to scale. In this study, it is shown that matrix of porous samples are weakly anisotropic as a result of inter-crystalline pores. At centimeter-scale, anisotropy can also be related to: (1) patchy distribution of some physical properties, (2) local cracks distribution, and (3) thick single fractures. The lack of correlation between stiffness components from seismic-scale measurements, and laboratory to multi-meter scale ones emphasizes the fact that, when fracturing dominates, measured anisotropy is dominated by fracture/fault related anisotropy and matrix-related anisotropy may be lost. So that, scale effect must be handled carefully in anisotropy analyses, especially for carbonate formations.
This paper has been withdrawn from the Technical Program and will not be presented at the 87th SEG Annual Meeting.
Keywords: reservoir characterization, ultrasonic, VTI, carbonate, anisotropy
Knowledge on hydraulic characteristics of fractured crystalline basement aquifers is limited resulting to lack of reliable information for effective groundwater management. This is crucial as these aquifers provide good sources of potable water in most rural communities. Heterogeneous nature of these aquifers requires detailed understanding and accurate estimation of hydraulic characteristics. This study estimated hydraulic characteristics of a fractured crystalline basement aquifer and inferred their influence on groundwater storage potential and flow. Aquifer Test Solver was used for automatic curve matching to identify appropriate aquifer models and test solutions for estimating hydraulic characteristics. Root-mean-square error (RMSE), mean absolute error (MAE) and correlation coefficient (R) were used to evaluate the performance of the fitted models. Plotted derivative curves were used to identify the presence of fracture dewatering in the aquifer. The R, RMSE and MAE ranged from 0.702 to 0.995, 0.31–3.45 m and 0.23 to 3.06 m, respectively. The fits between measured and estimated drawdowns were good and performance mostly acceptable and comparable to those of related studies. Storativity, transmissivity and hydraulic conductivity ranged from 0.0003 to 0.0680, 0.78 to 12.3 m²/day and 8.56 × 10⁻⁷ to 5.32 × 10⁻⁶ m/s, respectively. The storage potential of the aquifer varied from low to high, ability to transmit water per unit area was mostly medium though the ability to transmit water through its entire thickness was mostly low. The study area is dominated by leaky aquifer and fracture dewatering. Groundwater use should be monitored and managed effectively to avoid fracture dewatering and its associated risks.
Le radar géologique est une méthode d'investigation géophysique basée sur la propagation d'ondes électromagnétiques dans le sous-sol. Avec des fréquences allant de 5 MHz à quelques GHz et une forte sensibilité aux propriétés électriques, le géoradar fournit des images de réflectivité dans des contextes et à des échelles très variés : génie civil, géologie, hydrogéologie, glaciologie, archéologie. Cependant, dans certains cas, la compréhension fine des processus étudiés dans la subsurface nécessite une quantification des paramètres physiques du sous-sol. Dans ce but, l'inversion des formes d'ondes complètes, méthode initialement développée pour l'exploration sismique qui exploite l'ensemble des signaux enregistrés, pourrait s'avérer efficace. Dans cette thèse, je propose ainsi des développements méthodologiques par une approche d'inversion multiparamètres (permittivité diélectrique et conductivité), pour des configurations en transmission, en deux dimensions.Ces développements sont ensuite appliqués à un jeu de données réelles acquises entre forages.Dans une première partie, je présente tout d'abord la méthode numérique utilisée pour modéliser la propagation des ondes électromagnétiques dans un milieu 2D hétérogène, élément indispensable pour mener à bien le processus d'imagerie. Ensuite, j’introduis puis étudie le potentiel des méthodes d’optimisation locale standards (gradient conjugué non linéaire, l-BFGS, Newton tronqué dans ses versions Gauss-Newton et Exact-Newton) pour découpler la permittivité diélectrique et la conductivité électrique. Je montre notamment qu’un découplage effectif n’est possible qu’avec un modèle initial suffisamment précis et la méthode la plus sophistiquée (Newton tronqué). Comme dans le cas général, ce modèle initial n’est pas disponible, il s’avère nécessaire d'introduire un facteur d'échelle qui répartit le poids relatif de chaque classe de paramètres dans l'inversion. Dans un milieu réaliste avec une acquisition entre puits, je montre que les différentes méthodes d'optimisation donnent des résultats similaires en matière de découplage de paramètres. C'est finalement la méthode l-BFGS qui est retenue pour l'application aux données réelles, en raison de coûts de calcul plus faibles.Dans une deuxième partie, j'applique cette méthodologie à des données réelles acquises entre deux forages localisés dans des formations carbonatées, à Rustrel (France, 84). Cette inversion est réalisée en parallèle d'une approche synthétique à l'aide d'un modèle représentatif du site étudié et des configurations d'acquisition similaires. Ceci permet de pouvoir comprendre, contrôler et valider les observations et conclusions obtenues sur les données réelles. Cette démarche montre que la reconstruction de la permittivité est très robuste. A contrario, l'estimation de la conductivité souffre de deux couplages majeurs, avec la permittivité diélectrique, d'une part, et avec l'amplitude de la source estimée, d'autre part. Les résultats obtenus sont confrontés avec succès à des données indépendantes (géophysique depuis la surface, analyse sur échantillons de roche), et permet de bénéficier d'une image haute-résolution des formations géologiques. Enfin, une analyse 3D confirme que les structures 3D à fort contraste de propriétés, telles que la galerie enfouie sur notre site, nécessiteraient une approche de modélisation 3D, notamment pour mieux expliquer les amplitudes observées.
SOME NEW Q–VALUE CORRELATIONS TO ASSIST IN SITE CHARACTERIZATION AND TUNNEL DESIGN
N. Barton
Nick Barton & Associates, Fjordveien 65c, 1363 Høvik, Norway
Abstract
The rock mass quality Q-value was originally developed to assist in the empirical design of tunnel and cavern reinforcement and support, but it has been used for several other tasks in rock engineering in recent years. This paper explores the application of Q and its six component parameters, for prediction, correlation and extrapolation of site investigation data, and for obtaining first estimates of some input data for both jointed distinct element and continuum-approximation modelling. Parameters explored here include P-wave velocity, static modulus of deformation, support pressure, tunnel deformation, Lugeon-value, and the possible cohesive and frictional strength of rock masses, undisturbed, or as affected by underground excavation. The effect of depth or stress level, and anisotropic strength, structure and stress are each addressed, and practical solutions suggested. The paper concludes with an evaluation of the potential improvements in rock mass properties and reduced support needs that can be expected from state-of-the-art pre-injection with fine, cementicious multi-grouts, based on measurements of permeability tensor principle value rotations and reductions, caused by grout penetration of the least favourable joint sets. Several slightly improved Q-parameter ratings form the basis of the predicted improvements in general rock mass properties that can be achieved by pre-grouting.
The Hoek-Brown failure criterion for rock masses is widely accepted and has been applied in a large number of projects around the world. While, in general, it has been found to be satisfactory, there are some uncertainties and inaccuracies that have made the criterion inconvenient to apply and to incorporate into numerical models and limit equilibrium programs. In particular, the difficulty of finding an acceptable equivalent friction angle and cohesive strength for a given rock mass has been a problem since the publication of the criterion in 1980. This paper resolves all these issues and sets out a recommended sequence of calculations for applying the criterion. An associated Windows program called " RocLab " has been developed to provide a convenient means of solving and plotting the equations presented in this paper.
The large-scale equivalent permeabilities of strike-slip faults in porous sandstone are computed from detailed field measurements. The faults, which occur in the Valley of Fire State Park, Nevada, were previously characterized, and the flow properties of their individual features were estimated. The faults formed from the shearing of joint zones and are composed of a core of fine-grain fault rock (gouge) and deformation bands and a peripheral damage zone of joints and sheared joints. High-resolution fault-zone maps and permeability data, estimated using image analysis calibrated to actual measurements, are incorporated into detailed finite difference numerical calculations to determine the permeability of regions of the fault zone. Faults with slips of magnitude 6, 14, and 150 in are considered. The computed fault-zone permeabilities are strongly anisotropic in all cases. Permeability enhancement of nearly I order of magnitude (relative to the host rock) is observed for the fault-parallel component in some regions. Fault-normal permeability, by contrast, may be 2 orders of magnitude less than the host rock permeability. The fault-normal permeability is a minimum for the fault with the highest slip. For a representative fault region, the fault-parallel component of permeability is highly sensitive to the fracture aperture, although the fault-normal permeability is insensitive. The procedures developed and applied in this article can be used for any type of fault for which detailed structural and permeability data are available or can be estimated.
We have studied the geometry and continuity of structures and diagenetic features of a normal growth fault in poorly lithified sediments. Fault-zone width and complexity vary spatially with the grain-size distribution of faulted beds. The fault zone is narrow and structurally simple where it cuts either thick beds with >20% clay and silt, or thin beds that alternate between >20% and ≤20% clay and silt. Where the majority of beds juxtaposed by the fault are ≥80% sand and gravel, and clay beds are thin and rare, the fault zone is wide and structurally complex. In all cases, the fault zone can be divided into three architectural elements. The core includes the primary slip surface(s) and a nearly continuous clay smear 0.3 - 32 cm wide. It is flanked by structurally and lithologically heterogeneous mixed zones, which include material derived from adjacent sediments during fault movement. Mixed zone sediments vary from little deformed to well foliated, tectonically mixed material within which bedding has been destroyed. The mixed zones are bound by damage zones, within which deformation was confined to minor faults and folds. Grain-size and structural variations among these elements lead us to conclude that they have hydrologie significance. In addition, the fault zone is preferentially cemented with respect to adjacent sediments. We use degree of cementation as a proxy for fluid flux, and patterns of cementation as a record of paleo-flow pathways. Extensive sparry calcite cement is typically confined to coarse-grained sediments in the hanging wall (basinward) mixed zone. Steeply plunging, elongate patterns of cement are interpreted to record subvertical groundwater flow at the time of precipitation. As regional flow is inferred to have occurred roughly from the margins to the center of the basin at the time of cementation, these relationships indicate a combination of crossfault and subvertical, fault-parallel flow.
